Honestly, I might be the only person underwhelmed with II. It sounds like II is great as an exchange company, and if all you want to do is exchange your week that's great. RCI is much better IMHO as a cash Extra Vacations / Last Call company. The problem with an exchange is you have to have a week you are locked into paying for every year, the benefit of the Extra Vacations / Last Call is you can add weeks usually for less than MFs as you go - it's a great augmentation to owned weeks in any mini-system. I don't think you're likely to find exchanges that are a better value outside most of the mini system once you take in the $200+ exchange fee from both.
That said, Spinnaker I don't know of like a system, so if it's a more traditional timeshare, exchanging the week can be great with II.